In January, 1999, the Johns Hopkins University Applied Physics Laboratory (APL) released a Global Positioning Satellite (GPS) risk assessment study. This study was carried out because GPS is being evaluated for possible use as the sole basis for airplane navigation in U.S. national airspace. The study concluded that the only significant risk was interference (intentional or unintentional). The study went on to outline three primary sources of unintentional interference, which are: (1) out-of-band television transmissions, (2) very high frequency (VHF) broadcasts, and (3) over-the-horizon (OTH) military radar. The study recommended the use of antenna nulling and signal processing techniques to overcome any kind of interference.
One specific of the use of GPS for airplane navigation is that GPS has become a significant, enabling technology for present and future military fighter jets. GPS technology is being utilized for many other military aspects as well, and is forming the foundation for new paradigms in military tactics. As a result, the U.S. military is growing increasingly reliant on GPS.
The success of a mission and the lives of troops can often depend on the accuracy and availability of a GPS system. Therefore, signal denial is a major concern, whether it is caused intentionally or unintentionally. For this reason, many military GPS systems use specialized antenna arrays, called controlled reception pattern antennas (CRPAs), which can work in conjunction with integrated electronics to adaptively cancel (or null) unwanted signals. These antennas and the associated electronics are often referred to generically as anti-jam antenna systems.
Many branches of the military are now deciding to add anti-jam capabilities to their existing GPS systems. This trend will carry over to many commercial/civil organizations in the near future. Beginning with GPS satellites planned for launch in 2005, two additional commercial signals will be available. The first signal, which is designated L5, will be broadcast over a carrier frequency of 1176.45 MHz. The second signal will be added to the existing L2 carrier frequency. These new signals are intended to modernize the GPS standard by facilitating higher accuracy and availability for critical commercial applications, including primary aircraft navigation, life-saving rescue operations, and so forth. In time, these critical civilian applications will require system availability and accuracy comparable to that required by the military today.
The addition of anti-jam capability to existing military GPS systems will, in many cases, present the issue of replacing existing single-element Fixed Reception Pattern Antennas (FRPAs), which are relatively small, with a larger CRPA array and also an antenna electronics (AE) unit. With existing physical and budgetary constraints, array size and cost will be prime discriminators during procurement for this transition.
Most existing CRPA array designs use microstrip patch antenna elements. These elements are attractive because they are relatively simple in design, exhibit a low profile, and have performance characteristics which are well understood. The existing state-of-the-art CRPA arrays (which utilize patches) can provide the anti-jam performance required for most, if not all, of the projected military applications. However, the size and cost of existing CRPA arrays will be a significant impediment to their proliferation. In this regard, existing 7-element GAS-1 CRPA arrays are typically about 14 inches in diameter, and can each cost as much as $12,000.
Existing GAS-1 antennas are too large to fit on many aircraft, and there is a need for a smaller array that will meet many or all requirements of the U.S. military. In order to provide a smaller array, it is necessary to shrink the size of the antenna elements, and/or position them closer together. On the other hand, to be an efficient radiator, an antenna element typically needs to be a certain size. For example, a single antenna element normally needs to be at least a quarter wavelength to a half wavelength in size in order to operate efficiently. An array would need to be larger. In fact, electrically small antennas usually exhibit degraded performance (reduced bandwidth, efficiency, and so forth), which can greatly hinder the overall system performance. An electrically small antenna which is efficient and broadband is highly desirable.
It is possible to make existing CRPA patch elements physically smaller by using a substrate material which has a high dielectric constant (loading). These smaller loaded elements can, in theory, be positioned more closely together, in order to form a smaller array. However, there are performance and other sacrifices associated with this approach. First, the dielectric loading tends to decrease the element's bandwidth and efficiency, and to increase the element's weight and cost. Narrow bandwidth coupled with manufacturing and material tolerances can lead to significant production problems, and can ultimately drive up the cost of the array. Second, testing has shown that small loaded patch elements can exhibit very high element-to-element coupling values when they are separated by a distance of one-third wavelength or less. This increased coupling leads to degraded anti-jam performance.
Consequently, there is a need for a cheaper and smaller CRPA array which provides acceptable anti-jam performance and which is less dependent on manufacturing and material variations. Some associated technical problems are that unloaded broadband antenna elements tend to be large (the opposite of what is needed), and the close element-to-element spacing needed for a reduced-size array tends to degrade the performance of the antenna and inhibit the overall system's ability to cancel the interfering or jamming signals.